When a structural container is produced by welding, deformation and a residual stress occur due to a welding heat. For this reason, it is difficult to produce a structural container having high precision. Particularly when a welding heat input is high, its influences become great. Once the container undergoes deformation, machining becomes necessary to correct the shape, and the reduction of deformation has been therefore desired. Arc welding such as carbonic acid gas arc welding or MIG welding has greater welding heat input than high density energy welding such as laser welding or electron beam welding. Accordingly, weld deformation and the residual stress tend to remarkably increase when the welding quantity becomes great. Since the welding speed is low, productivity is low, and large quantities of fumes and spatters are generated. Therefore, an extra process step is necessary for removing them and at the same time, the environment of the work is deteriorated.
The application of the high density energy welding methods such as laser welding and electron beam welding has been vigorously studied.
On the other hand, the high density energy welding method such as laser welding and electron beam welding, can make the heat input smaller than the gas arc welding and MIG welding methods, and the application of this method to welding of different kinds of materials has been studied, too. When the different kinds of materials such as a carbon steel and an austenitic stainless steel are bonded by welding, the weld metal formed at the joint portion has intermediate components between these different kinds of materials. At this time, there is the case where a martensitic structure having lower brittleness or an intermetallic compound is formed due to mixture of components of both materials. Particularly, the weld boundary on the carbon steel side comprises a metallic structure containing the intermetallic compound between carbon and the Cr element and an extremely hard and brittle martensitic structure. Because the martensitic structure is likely to occur at the weld portion of the different kinds of materials, the resulting weld becomes hard, its mechanical properties drop, and various weld defects such as high temperature crack and low temperature crack are likely to occur. According to the welding method involving a large heat input such as the carbonic acid gas arc welding or MIG welding method, the brittle martensitic structure and the intermetallic compound are likely to be formed. To obtain a weld portion having high soundness and reliability, it is essentially necessary to control the composition and structure of the weld metal at the joint portion by, for example, carrying out build-up welding of a suitable welding material on the carbon steel side. When a high density energy welding method having a small heat input quantity such as laser welding or electron beam welding method is used, the welding width becomes Small and the depth of penetration becomes great. Because the melt quantity becomes small, it is expected that the absolute formation quantity of the brittle martensite structure and the intermetallic compound can be limited. Accordingly, it is expected that this method can obtain a weld portion having sufficiently high soundness and reliability by adding a small amount of a suitable welding material.
In the case of a vane member and a spindle member of a hydraulic turbine for power generation for which high reliability is a requisite, a 13 Cr stainless steel is used for the vane member which comes into contact with water, and a carbon steel is used for the spindle member to which corrosion resistance can be imparted by a lubricant, etc. Japanese Patent Laid Open No. 132378/1983 discloses a method which uses electron beam welding for welding of this vane member and the spindle member and regulates the composition of the joint portion by interposing an intermediate medium metal of a pure nickel material. According to this method, a weld portion having higher impact toughness than welding without using an addition metal can be obtained.